Oil-well steel pipes are used as a casing or a tubing for an oil well or a gas well. An oil well and a gas well are hereinafter referred to simply as an “oil well”. With increasing depth of oil wells, oil well pipes are required to have a higher strength. Conventionally, oil well pipes of 80 ksi grade (having a yield stress of 80 to 95 ksi, that is, 551 to 654 MPa) or 95 ksi grade (having a yield stress of 95 to 110 ksi, that is, 654 to 758 MPa) have been mainly used. However, there has recently been increasing use of oil well pipes of 110 ksi grade (having a yield stress of 110 to 125 ksi, that is, 758 to 861 MPa).
Many of the deep oil wells recently developed contain corrosive hydrogen sulfide. In such an environment, if the strength of steel is increased, the susceptibility of steel to sulfide stress cracking (herein after referred to as “SSC”) is increased. In the case of increasing the strength of an oil well pipe used in an environment containing hydrogen sulfide, therefore, a higher sulfide stress cracking resistance (SSC resistance) is desirable.
For example, techniques described below have been proposed as a technique to improve the SSC resistance of steel. The proposed techniques include:
increasing the proportion of martensite in the steel micro-structure to 80% or higher;
performing tempering on steel at a high temperature to make carbides in steel spherical;
acquiring higher cleanliness of steel;
making the steel micro-structure finer; and
suppressing the diffusion coefficient of hydrogen and the dislocation density in steel.
Further, the SSC resistance of steel can be improved by controlling nonmetallic inclusions. JP2001-131698A (Patent Document 1), JP2004-332059A (Patent Document 2) and JP2001-73086A (Patent Document 3) propose techniques to improve the SSC resistance by controlling nonmetallic inclusions.
Patent Document 1 discloses particulars described below. In a case where Ti nitride is coarsely formed even in a low alloy steel, the Ti nitride coarsely formed acts as a pitting starting point. Generation of pitting induces SSC. Therefore the Ti nitride is made finer. In this case, the SSC resistance of the steel is improved.
Patent Document 2 discloses particulars described below. In a low alloy steel, Nb-based inclusions satisfying the following expression are contained at a rate of ten or more particles per 1 mm2 sectional area.aNB×bNB≦150
In this expression, aNB is the length of the major axis (μm) of a Nb-based inclusion, and bNB is the content (mass %) of Nb in the Nb-based inclusion. Nb-based inclusions having aNB smaller than 1 μm are ignored. In this case, the occurrence of pitting in the low alloy steel is suppressed and the SSC resistance is improved.
Patent Document 3 discloses particulars described below. The chemical composition of a steel satisfies the following expression:(1+4.3×[V]+3.4×[Nb]+2.5×[Ti])/(7.8[Cr]×[Mo])>1
For [V], [Nb], [Ti], [Cr] and [Mo], the contents of the corresponding elements (in wt %) are substituted. In this case, the generation of M23C6 carbide is suppressed. Further, the SSC resistance of the steel is improved by making the carbide spherical.